If it seems to do it at a certain speed, regardless of engine rpm (i.e. in neutral or any other gear), then it could be a front wheel bearing. This will get noticeably worse in time.

If it does it more in a certain gear (usually OD at a decent speed), then it is more likely gear whine. You'll probably always have at least a little bit of gear whine, it's just a matter of how loud it is and whether or not it is noticeable.